The present invention relates to a maleimide-based copolymer having high heat resistance and impact resistance, and showing a superior molding property and thermal stability, because of a fact that, when an aromatic vinyl-based monomer and a maleimide-based monomer is copolymerized, the content of slide products formed via a Diels-Alder type reaction is little.
The present invention relates to a process for producing a maleimide-based copolymer, which depresses greatly the amount of a compound (X) forming via the Diels-Alder type reaction between the aromatic vinyl-based monomer and the maleimide-based monomer existing as a side reaction in a copolymerization reaction of the aromatic vinyl-based monomer with the maleimide-based monomer.
The present invention relates to a thermoplastic resin composition which was improved in various properties such as heat resistance and so forth by combining a maleimide-based copolymer of the above type with other thermoplastic resins.
It has been known that the maleimide-based copolymer is a thermoplastic resin having a high heat deflection temperature and a high thermal decomposition temperature and, in general, the copolymer is used to elevate heat resistance, impact resistance and molding properties of articles in which other thermoplastic resins are used.
The maleimide-based copolymer is produced by usually copolymerizing the maleimide-based monomer and the aromatic vinyl-based monomer as well as, if necessary, another monomer.
Physical properties of the maleimide-based copolymer are affected by proportion of the maleimide-based monomer unit in structure and, if the proportion is low, heat resistance becomes low, so that a resin composition may not obtain excellent heat resistance and also, if the proportion is too high, molding and processing properties becomes bad and impact resistance may decrease.
A copolymer having a relatively small proportion of the maleimide-based monomer unit is generally prepared by emulsion polymerization or suspension polymerization. According to the emulsion polymerization, a maleimide-based copolymer having a high maleimide-based monomer unit is hard to soften and very difficult to recover from an emulsified solution and, therefore, a copolymer having a low proportion of the maleimide-based monomer unit has been produced. Then, a copolymer obtained from the emulsion polymerization shows bad impact resistance by an effect of a residual emulsifier etc, and coloring in the course of molding is very much. In the case of suspension polymerization, the aromatic vinyl-based monomer and maleimide-based monomer is liable to form an alternating copolymer and, therefore, when a copolymer having a high proportion of the maleimide-based monomer unit is desired to obtain, a heterogeneous copolymer is easy to form. In the solution polymerization and bulk polymerization, there have been carried out several attempts to obtain a copolymer in which a unreacted maleimide-based monomer is decreased by a maturing reaction or a polymer in which component distribution in the molecular weight division is narrow by using a defined polymerization process and the unreacted maleimide-based monomer is small in amount (Japanese Official Patent Provisional Publications, heisei 2-51514 and 3-205411 ) .
When the maleimide-based monomer remains in the maleimide-based copolymer, the monomer is easy to become origin of coloring a molding or polluting a metal mold and also, it decreases heat resistance. Because of this, several -treatments have been carried out to decrease an residual amount of the unreacted maleimide-based monomer.
The suspension polymerization and polymerization have been adopted to obtain a maleimide-based copolymer having a high proportion of the maleimide-based monomer unit and being useful as a resin improver, however, even if the unreacted monomer is removed after polymerization, a copolymer inferior in physical properties such as heat resistance and impact resistance is formed.
Resin articles using a copolymer of this kind are not sufficiently elevated in mechanical strength and heat resistance.
According to inventors' studies, when a maleimide-based copolymer is made by carrying out a radical polymerization reaction between the maleimide-based monomer and the aromatic vinyl-based monomer, it was found that; the compound (X) is formed as a side product via a Diels-Alder type reaction between the maleimide-based monomer and the aromatic vinyl-based monomer. The inventors suspect the reaction course of, for example, N-phenylmaleimide with styrene as follows: The aromatic vinyl-based monomer (a) and a first maleimide-based monomer (b) makes an intermediate by the Diels-Alder type reaction as shown in the following formula (II) and this intermediate undergoes a reaction with a second maleimide-based monomer (b) to make the compound (X) as shown by the following formula (III). ##STR2##
The inventors studied extensively an effect of the compound (X) upon properties of the maleimide-based copolymer and, as a result, found that, if the maleimide-based copolymer contains the compound (X), the heat resistance decreases as well as the impact resistance decreases. Since a rate forming the compound (X) becomes faster with an increasing amount of the maleimide-based monomer used in polymerization, there is a trend that a content of the compound (X) increases with an increasing proportion of the maleimide-based monomer unit in the maleimide-based copolymer.
Accordingly, it is required to obtain a maleimide-based copolymer having a low content of the compound (X).
The compound (X) is not eliminated under a condition that the unreacted monomer is removed, but it remains and, therefore, it is necessary to think up a new process in order to decrease the content of compound (X) in the copolymer.
From a view point of avoiding increase of the unit process in after-polymerization, not dropping productivity, and not enlarging the distribution of composition and molecular weight in the copolymer, the inventors investigated a polymerization process which decrease the amount of compound (X) being produced as a side product in the course of the polymerization reaction as much as possible.